Question

A buffer solution that is 0.325 M in HCN and 0.325 M in KCN has a pH of 9.40. Addition of which of the following would increase the capacity of the buffer for added H3O+?

both HCN and KCN

pure water

HCN

KCN

None of the Above

One or more of these choices is the answer, please answer in that format. I have already asked this question and got the wrong answer.

A buffer solution is made that is 0.388 M in HCN and 0.388 M in NaCN.

(1) If K_a for HCN is 4.00×10^-10, what is the pH of the buffer solution?

(2) Write the net ionic equation for the reaction that occurs when 0.114 mol NaOH is added to 1.00 L of the buffer solution.

The last person who answered this didn't get it right.

Answer 1

A buffer solution that is 0.325 M in HCN and 0.325 M in KCN has a pH of 9.40. Addition of which of the following would increase the capacity of the buffer for added H3O+?

Capacity of buffer for H3O+, i.e., what do we need to do to increase change of pH when H+ addition:

both HCN and KCN

pure water --> Not really, since this will only dilute the system, but the ratio of acid/base remains the same

HCN --> this will NOT help, since HCN is acidic, and H+ is acidic as well

KCN --> This WILL help, since CN- will neutralize H+ addition to form HCN

None of the Above --> false, KCN is true

Q1.

for

HCN + NACN, this is a buffer so

pH = pKa + log(NaCN/HCN)

pKa = -log(Ka) = -log(4*10^-10) = 9.397

pH = 9.397 + log(0.388/0.388)

pH = 9.397

Q2.

Net ionic when we add

NAOH to the buffer:

base will react with acid so

NAOH(aq) + HCN(aq) --> H2OP(l) + NaCN(aq)

net ionic includes only those in that react in solution

H+(aq) + OH-(aq) --> H2O(l)